Osmosis 3.1 and 3.2 Flashcards
What is osmosis?
Net movement of H2O from regions of high H2O concentration to regions of low H2O concentration
What are the differences between Osmosis and diffusion?
Net movement of SOLUTE from regions of high SOLUTE concentration to regions of low SOLUTE concentration
Water can move freely between cells and the ECF so that the body is in osmotic equilibrium. Not all solutes move freely.
ALL H2O movements in the body are passive, (via aquaporins, protein water channels).
H2O concentration is inversely related to the concentration of solute, ie the more solute particles there are in solution, the more they will displace H2O molecules lowering the concentration of H2O.
Diffusion through a freely permeable membrane , water and glucose example?
If a solution of different concentration is separated by a membrane permeable to the H2O and solute then after a time will end up with equal concentration and equal volume either side of membrane = DIFFUSION!
Diffusion through a selective membrane, water and glucose example?
If a solution of different concentration is separated by a membrane permeable to H2O only, then after a time will end up with equal concentration either sided of membrane but different volumes. Assumes compartments are expandable.
What is osmotic pressure?
If we try to oppose this increase in volume the pressure required to do so is known as the osmotic pressure = the P required to prevent water movement
When is it diffusion and when is it osmosis?
Where we have diffusion we also have osmosis (water and solute move, in opposite directions).
Where we have osmosis, we may or may not have diffusion – if the membrane is only permeable to water, then water moves but the solute does not (so no diffusion).
Osmosis with no diffusion will result in a change in cell volume!
Why are cell membranes semi permeable membranes?
Cell membranes act as semipermeable membranes. They are permeable to H2O and gases but some molecules in the ECF and ICF are unable to cross the membrane.
Which wall is permeable to ions?
Ions pass freely across the capillary wall so exchange readily between plasma and ISF but they do not penetrate the cell membrane hence differential distribution between ECF and ICF.
What is concentration measured in?
concentrations are expressed as molarity or mol/l and a mole consists of 6.02 x 1023 molecules.
What determines the osmotic effect on H20?
However it is the number of solute particles, NOT molecules, which determine the osmotic effect on [H2O] and this can be misleading because some molecules dissociate in solution.
What does osmolarity measure?
measures the concentration of biological solutions in units of “OSMOLES” and describes the number of particles/L of solution, (in reality use milliosmoles, because biological solutions are so weak).
What is the osmolarity of normal human plasma and within cells?
285 mOsmol/l
-often taken as 300
What does osmolarity tell us about the particles?
Osmolarity ONLY describes the number of particles in solution, it says nothing about the NATURE of the particles, critically it does not tell us if the particles can cross cell membranes.
What is an osmotic flux?
If a solute cannot cross the membrane, any change in its concentration produces an osmotic flux, causing a net movement of water in one direction or the other, which changes the cell volume.
What does the volume of a cell always depend on?
the volume of a cell at any time is dependent on the concentration of non-penetrating solutes on the either side of the membrane.
